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When you scale an image to make it fit a different aspect ratio, all the pixels in the image are affected and stretched uniformly. Although distorting a sky or a grassy field when adjusting an image to improve composition or fit a layout may not be noticeable, you cannot easily scale images with people or recognizable objects.
Photoshop now includes the Content-Aware scaling feature so you can more easily upscale or downscale an image, or even change its orientation, without distorting the main subject.